The present invention relates to a device for controlling an apparatus which consumes mains electric power to be responsive to changes in demand on the mains supply.
To perform their function of safely and reliably distributing electricity from generators to consumers, alternating current electricity grids need to have control systems that keep supply (generation), and demand (load) in very precise balance. The system frequency is the signal by which this control is achieved. The system frequency, usually 50 Hz, 60 Hz or 400 Hz, synchronizes all generation and synchronous load on the system, and varies according to the imbalance. If the load is too high, the system frequency slows down and vice versa. As load is constantly varying, the system frequency also fluctuates, although mechanical inertia in the system limits the rate at which the frequency can change.
The frequency of the system as a whole is influenced by the overall mix of generation and load on the system. Much of the load is ‘resistive’, e.g. electrical lights, with the load varying according to the voltage at the point of the load. These types of load do not respond directly to changes in the frequency of the system, unless those changes also involve voltage changes.
The system will also have load which is ‘synchronous’, e.g. electric motors whose speed is locked to the frequency of the system as a whole. When the frequency of the system drops, much of this load actually reduces the energy it consumes, because it goes slower. Thus, the system, when heavily loaded, has an innate ability to respond to demand and, thus, frequency changes, in a useful way.
However, the power generators also have an innate tendency to reduce their output when the frequency of the system drops. This can, therefore, lead to a vicious circle and, unless corrected, leads to unstable operation.
Methods of monitoring this tendency are known, but the conditions associated with this monitoring can be onerous. The conventional frequency control method has been to fit governors onto generators, so that, if the frequency drops, they increase their output, and if the frequency rises, they reduce their output. This works, but relatively slowly. It can take minutes for a generator to fully increase its output in response to a change in frequency, and during this time the frequency is depressed. So quite wide and unpredictable fluctuations in the grid frequency are the norm.
Headroom to increase capacity involves generating capacity that is usually unused and this implies less efficient running of the generator. Headroom to reduce energy is less of a problem, but problems can arise when demand is very low and the baseload plant is already running at minimum capacity.
To retain stability, the system as a whole needs plant which is able to respond automatically with additional dispatch energy essentially equivalent to the largest credible loss of generation (in reality, actually taking into account factors such as the behaviour of the synchronous load, the behaviour of the generation plant and the total load on the system). The largest credible loss of generation is usually considered to be the largest single power unit running. In England and Wales, this is normally Sizewell B at 1.2 GW, but the French Interconnector is also two inputs of 1 GW each. This additional energy must be available before the frequency actually drops below the control limit. In the UK, inertia in the power supply system gives about 10 seconds before limits are breached. Major loss scenarios are considered exceptional, and it is considered acceptable to range beyond the normal 1% frequency deviation limit. Ensuring that this capacity is available is extremely important and is a major consideration and complication in the overall running of the electric supply system. This concern is normally met by holding contracts to pay generators to have governors and to switch them on when requested, and/or by scheduling plant to be only partially loaded.
The service of maintaining the system frequency is known as Response, and in all grids, it is a responsibility of the grid operator to ensure the system as a whole has sufficient Response available to handle short term contingencies. As well as paying generators to have their generators operational, this will often involve purchasing “headroom” so that generators can increase (or reduce) their output when necessary. To ensure grid stability, there must be enough Response available to compensate for possible losses of generation and/or load. If a generator (or the transmission line from it) has a fault, and stops generating, other generation must replace it, generally within a few seconds. If load suddenly stops (as in a power cut across an area), then Response must reduce the generation by an equivalent amount.
If available Response is inadequate, the frequency will not stabilise. If the frequency moves beyond limits, then this will damage much of the equipment connected to the grid. Generators will overspeed (and eventually blow up). Motors will overheat or stall, and other equipment will be damaged. To prevent long term damage, the grid has a variety of “frequency sensitive relays” that look at the frequency and its rate of change, and, if predefined limits are exceeded, will disconnect a portion of the grid or the more local distribution system. So if, for example, the frequency is dropping, these devices will progressively disconnect areas of the country until the frequency stabilizes. Load shedding usually starts automatically before the system frequency drops below 48.5 Hz, i.e. 1.5% less than the controlled limit.
The resulting area blackout is undiscriminating, in that high value uses, such as hospitals and train lines lose power at the same time as less critical loads, such as street lighting or domestic consumption.
If, on the other hand, the frequency is rising, other devices will disconnect generators.
This has to be done automatically, as damage to grid and electrical equipment can occur within sub-seconds of faults arising. Any human intervention is too slow. Grid operators often have prior arrangements with industrial consumers of large loads so that less essential equipment is disconnected before it becomes necessary to disconnect larger areas.
Under some circumstances, cascades of failures can occur. If load is lost, frequency rises, so generation is cut off, so load again exceeds generation and more load is lost. If the control systems are inadequate, large scale blackouts can occur within seconds of a first fault, and we have seen this in the recent blackout in the East Coast of the US. More commonly, as we saw in London, failures are contained to smaller areas.
Once a large scale blackout has occurred, recovery is slow. To start up a generator generally requires generators to have some power available to do so. If no power is available, they cannot start. So grid systems have services, known as “Black Start” services, whereby a subset of generation has the capacity to start and continue generating, even when the rest of the grid is inactive. Grid operators have pre-planned sequences for restoring generation and load. These ensure that the limited initial supplies are used first to provide communication and control, then to start up bigger generators, and thereafter load is progressively connected to match the increasing availability of generation. The entire process of black start is a fraught one. It is a very rare event, and not one that can be practised except in an actual crisis. Everybody involved is under severe pressure, and the systems are being operated quite outside their normal operating range (and sometimes outside their design range). Every step when load or generation is added is a shock to the system, and the grid can take seconds or minutes to stabilise after it happens. Sensible prudence would suggest making changes in small increments. This inevitably slows down the overall process, prolonging the blackout for those who have still to be reconnected.
The present invention aims to:    1. Enhance the Response services, stabilizing the grid before and during crisis, making it more resilient and reliable;    2. Enable much greater discrimination in the loads that are lost when crises arise, so that essential services (such as hospitals, trains and subways) are more likely to remain unaffected;    3. Minimize the extent of blackout during a crisis, so that smaller areas are affected, and larger areas continue to have electricity;    4. Soften the shocks to the system during the Black Start process. Larger loads and areas can be reconnected more quickly, so speeding recovery.
If introduced in a progressive long term way by embedding the inventive device in appliances as they are replaced, all this can be achieved at very low investment cost. Much can also be achieved in the shorter term, but at greater cost in retrofitting or in early retirement of existing equipment.